1. Field of the Invention
This invention relates to the production of high-permeability grain-oriented silicon steel, and in particular, it relates to the making of such steel which contains boron and is ductile, as is evidenced by the ability of the steel, in the form of strip or sheet to withstand a number of 180-degree bends before rupturing.
2. Description of the Prior Art
Those skilled in the art of making specialty steels are familiar with the making of grain-oriented silicon-containing electrical steels in the form of sheets of steel having a thickness of 14 mils or less, or in the form of a coiled strip of steel of such thickness, from which sheets may be cut. The product is used, for example, for the making of transformers, which may be of either the "wound" or the "stacked" type.
The known process or processes for the making of such product involve, as a first step, the melting of a steel of suitable chemical composition. In general, this is a steel which consists essentially of iron plus about 2.5 to 4 weight percent of silicon. Such steel is then solidified and subjected to a series of hot-rolling and cold-rolling operations, usually with intermediate annealings, given a decarburizing annealing, and then provided with a coating (typically mainly magnesium oxide, applied to the steel as a slurry and dried) and coiled and given a final texturing anneal, in which the desired grain-oriented texture is developed Thereafter, the steel is scrubbed to remove the separator coating, and the steel is then applied to its intended uses as indicated above.
There have developed in recent years certain approaches to the making of high-permeability, low-core-loss grain-oriented silicon steel, in accordance with which the steel is caused to contain either nitrides of aluminum and/or sulfides or selenides or in another approach, boron, alone or with nitrogen, as in U.S. Pat. Nos. 3,905,842 and 4,096,001. A practice of including boron in the magnesium oxide separator-material coating which is applied before the final texturizing anneal is disclosed in U.S. Pat. No. 4,096,001.
A practice of applying to the steel, after the final texturizing anneal, a coating based on magnesium oxide and also containing boron, and giving the so-coated steel a further heat treatment involving a slow cooling, to improve the core-loss properties, is the subject of a copending U.S. application, Ser. No. 852,058, filed Apr. 15, 1986.
It is known, for example, from an article entitled "High Resolution Investigation of Influence of Boron on Fine-Scale Intergranular Microstructure of 316 L Stainless Steel", by L. Karlsson and H. Norden, Stainless Steel '84 [Proceedings of Conference]Chalmers University of Technology, Goteborg, Sweden, 3-4 Sept. 1984, published by the Institute of Metals in London, that boron is particularly reactive with molybdenum to form a molybdenum boride, Mo.sub.2 B.sub.5. It is also generally known in metallurgy that tungsten behaves similarly to molybdenum when substituted therefor on a 2:1 weight basis.
The high-permeability grain-oriented silicon steels made from boron-containing steel have, prior to the present invention, been poor in ductility, when that property is measured in terms of an ability to withstand some number of 180-degree bends without breaking. Such steel tends to contain iron-boride particles at or near the surface, and such particles, which are very hard and in a matrix not much softer, tend to serve as crack-initiation sites when the steel is bent.
There has not been in the prior art, to the inventor's knowledge, any teaching or suggestion to the effect that molybdenum and/or tungsten or any compound thereof be used as a component of the refractory-oxide coating composition, particularly one based on magnesium oxide, for the purpose of influencing favorably the ductility of the product steel.
There is a teaching, however, in Japanese Patent No. 44,395 of 1985, to the effect that it is desirable to add, in an amount of 0.1 to 10 weight percent, relative to the magnesium oxide, a fine metal powder, with the metal being selected from the group consisting of aluminum, silicon, titanium, chromium, zirconium, niobium, tin, tungsten, and/or molybdenum in order to improve the core-loss properties of the product silicon-steel sheets. This reference stresses the concept of adding the molybdenum or the like in the form of a fine powder of the free metal, as a way of adjusting the oxidation potential of the separating-material coating. The free metal acts as a getter, reacting with any oxygen or water which is released by decomposition of the magnesium oxide coating during the texturizing anneal. To those skilled in the art, this reference thus does not suggest that it would be useful to add to the separating-material coating any molybdenum or tungsten in the form of a compound of molybdenum or tungsten, such as molybdenum trioxide or tungstic acid, which is a compound with oxygen.